Turner syndrome (TS) occurs in 1:2000 live-born females, making it one of the most frequent chromosomal disorders and the only viable monosomy [1, 2]. Complete or incomplete monosomy X may cause detrimental problems pre- and postnatally, and affects overall survival [2]. The risk of premature death is increased threefold [1, 3], causing a significant reduction in life expectancy [2‐4], which is attributed to cardiovascular pathologies in over 50%. Congenital heart defects occur in 25–45% [5, 6] of all live-born TS females, with over-proportionately high numbers of left-sided heart defects such as bicuspid aortic valve (BAV), coarctation of the aorta (CoA), and hypoplastic aortic arch up to the full spectrum of hypoplastic left heart syndrome. The incidence of progressive aortic dilatation, the precursor of the potential fatal dissection, is increased and reported in 15–45% [7‐9] of all TS patients, depending on measurement techniques and definition. Aortic dissection is six times more frequent than in the general population, occurring in as early as the second decade of life [10, 11]. Risk factors for dilatation in the normal population are hypertension, BAV and CoA, one or more of which are present in nearly half of all TS patients [2]. However, nearly, 10% of TS females with aortic dissection present without risk factors [10, 12]. Thus, an intrinsic abnormality of vessel architecture has been presumed in the literature [13, 14]. On basis of this, the current study aimed to investigate the elasticity of the ascending aorta in pediatric TS as a surrogate marker of aortic wall texture abnormality and its correlation to valve morphology and aortic dilatation.

Patients were identified and recruited through a review of the institutional pediatric endocrinology and cardiology databases of the University Children´s Hospital Vienna. Inclusion criteria were genetically proven TS and age < 22 years. From the 50 patients fulfilling the requirements, 46 were eligible for this prospective echocardiographic investigation carried out between June 2014 and 2016.

A cohort of 46 healthy, age-matched females with tricuspid aortic valves (TAV), who had been referred to our institution for routine check-ups or who were part of the circle of the authors’ acquaintances, were prospectively recruited as controls [TAV group]. Moreover, ten non-syndromic females with isolated BAVs without further congenital heart diseases were selected during routine check-ups or from an earlier study [15], and included as a second control group [BAV group]. All subjects were in sinus rhythm and had normal left ventricular ejection fractions ≥ 60%. The study protocol was approved by the institutional review board of the Vienna University Hospital committee on human research. Mandatory written informed consent was obtained from probands, patients, and/or their relatives.

For all participants, weight and height measurements, age-related height z-scores [16], body mass indices, the corresponding percentiles, and body surface areas were obtained. Earlier interventions including cardiac operations and catheterizations, ongoing or previous growth hormone treatment and estrogens substitution were recorded.

Echocardiographic evaluation—performed by a single examiner (CP) using a Vivid E9 GE-Medical machine (Horton, Norway) with transducers from 1.5 to 12 MHz—included all standard views: M-mode measurement depicted left ventricular (LV) muscular mass and the dimensions of both ventricles. Valve function and morphology was judged in 4-, 5- and 2-chamber and short-axis view. Biplane ejection fraction was measured using Simpson´s method. The exact inner-edge to inner-edge diameters of the aortic valve (AV), aortic root (AoR), sinutubular junction (STJ), and ascending aorta (AA) 5–10 mm above the STJ were measured perpendicular to aortic flow in the parasternal long axis [17]. Valvular function was classified by color, PW- and CW-Doppler measurements. Aortic stenosis and regurgitation was scaled referring to the American Society of Echocardiography criteria [18].

BAV was defined as partial or complete fusion of 2 aortic valve leaflets with or without a raphe. Fusion of right- and non-coronary cusps formed LR-phenotype and fusion of right- and left-coronary cusps AP phenotype (schematic figure [15]). No left- and non-coronary cusp fusion was present in our patients. Z-scores were calculated according to the Warren data [19]; z-scores ≥ 2 were considered to be dilated.

For the elastic properties of the aortic root, 5–15 systolic (AoDS) and diastolic (AoDD) inner surface diameters were measured off-line using M-Mode imaging to compute mean values [15], whereas diastolic measurements were obtained ECG guided at the peak of the QRS complex and systolic diameters at the point of maximal anterior aortic wall movement. Blood pressure was scaled simultaneously (SBP systolic blood pressure and DBP diastolic blood pressure) using a Philips SureSignsVS2 (Andover, MA, USA). Stiffness index [20] was calculated using the following formula:

Distensibility [20, 21] was calculated using diameter and the area (AoAS systolic aortic area and AoAD diastolic aortic area) of the aortic root, respectively:

Normal elasticity ranges were established using the data of 115 control patients of ours. After exclusion of six outliers (all of them presenting with very low stiffness and very high distensibility levels, therefore, classified as normal elasticity), normal distribution of stiffness and distensibility either formula was established: mean values ± 1 SD (stiffness 3.96 ± 1.17 and distensibility 6.91 ± 1.99 cm²dynes⁻¹ × 10⁻⁶ and 56.2 ± 16.8 kPa⁻¹ × 10⁻³, respectively) were considered the upper and lower limits of normal, including 98% of our 46 female probands: A stiffness index > 5.13 and distensibility indices < 4.92 cm² dynes⁻¹ × 10⁻⁶ or < 39.4 kPa⁻¹ × 10⁻³ were considered pathological. There was no correlation between the stiffness and elasticity levels and age (stiffness r = − 0.142, p = 0.129, and distensibility r = 0.027, p = 0.781 and r = 0.039, p = 0.688, respectively); even after subdividing the probands into age-related groups of < 5, 5–10, 10–15, and > 15 years of age, no differences in the subgroups were detected by Kruska–Wallis test. Further on, no differences between male and female levels in the total group (p = 0.08, p = 0.23, p = 0.23, respectively) and in subdivided age groups were detected. The distribution of probands in the mentioned four age subgroups was balanced with n₁ = 32, n₂ = 22, n₃ = 33, and n₄ = 22.

In TS patients above 8 years of age, ambulatory 24-h blood pressure was registered (Mobil-O-Graph, Stoltenberg, Germany) and analyzed using the normative values for sex, age, and height from Soergel et al. [22]. A nocturnal dipping of systolic and/or diastolic values was considered abnormal if < 10%.

Data analysis was performed with IBM SPSS Statistics, version 23.0 (SPSS, Inc., Chicago). Data are presented as mean ± SD and are analyzed by parametric tests (unpaired t test) as well as non-parametric tests (Kruska–Wallis, Mann–Whitney U), depending on the outcome of a preceding Kolmogorov–Smirnov test. Correlation structures were tested using Pearson’s parametric correlation analysis. Odds ratios were calculated and the superadditive impact of influencing factors was investigated by multiple regression analysis. Intra-class correlation coefficients and percentage errors were calculated to ascertain the reliability of the echocardiographical examination. Data are considered statistically significant if p < 0.05 (bilateral). When required by the issue of multiple comparisons, significance levels were corrected by the Bonferroni–Holm method.

The findings of this study strongly suggest that TS presents with an intrinsic abnormality of the AA, which is measurable as early as in childhood by deterioration of its elasticity levels. These findings are neither caused by nor restricted to patients with aortic dilatation or BAV and/or CoA. Comparing TS patients with and without BAVs showed equal elasticity values, levels which were also measured in the subgroup of non-syndromic BAV patients. Even after exclusion of all patients with aortic dilatation and known dilatation risk factors, three of the remaining 20 females with TS had deteriorated stiffness and six and seven had pathologically reduced distensibility values, respectively.

To our knowledge, this is the first broadly based investigation showing a significant correlation of deteriorated aortic elasticity values to TS only in a wide age range spectrum of pediatric TS patients, especially after exclusion of patients with aortic dilatation, CoA and BAVs. Another study showing pathological elasticity levels in TS children independent of their aortic valve morphology [24] excluded adolescents due to a suspected negative influence of HRT as well as children below 6 years of age because of lesser cooperation, presenting a study population of only 15 patients. The same group presented an MRI study verifying these findings in a bigger study cohort, but including adult patients in a great amount as well [25]. Therefore, our study represents a more complete and greater cohort of children and adolescents. Other articles reporting on elasticity in TS chose elasticity measurements like pulse wave velocity or augmentation index [14, 26] which are both very much related to body size equations, thus being problematic in TS females due to their smaller stature and the known high frequency of elongation and tortuosity of their aorta, and which do not account for regional differences or abnormalities in arterial wall composition [7, 14, 27, 28].

Our study cohort depicts a frequency of risk factors for dilatation and dissection comparable to reports in the literature: The registered incidence of 30% of BAVs in our TS patients was expected based on earlier reports [7‐9, 27‐30]. Regarding both total incidence of CoA in 15% and occurrence in combination with BAV (36%) or TAV (6%), our results were also in line with earlier investigations [7, 8, 12, 27, 30]. Although our study cohort showed a low rate of confirmed hypertension in a 24-h measurement with 7% compared to earlier studies in adolescents, it displays comparable inadequate nocturnal dipping in 48% [7, 13].

Aortic diameter z-scores of the AoR, the STJ, and the AA were significantly larger in TS patients than in controls, although the mean diameters did not reach pathological levels. However, mean levels above a z-score of 2 were reached in the subgroup of TS with BAV at the AA, proving the known [7‐9, 31] highly significant correlation of BAV with aortic dilatation in our study group. Frequencies of dilatation of 15 up to 45% in TS patients have been reported [7, 8, 32]. This is in consensus with our data, which shows dilatation in 30% of our entire study group, and in still 16% of those with TAVs and in even 64% in the ones with BAVs. This equates to an almost 40-fold risk for a z-score > 2 in the latter subpopulation compared to our control group.

However, although dilatation of the AA is a well-known risk factor or even a precursor of the life-threatening rupture of the aorta, 10% of all dissections in TS happen in patients without risk factors [10, 12]. Therefore, we hope to assist in identifying these at-risk patients by their deteriorated elasticity measurements in the future.

The reasons for aortic wall abnormalities in TS have been speculated on. Deteriorated aortic elasticity can be brought on by the impeded function of either smooth vasculature or composition of the extracellular matrix, an impaired production, increased degeneration, or proteolytic degradation [33]. Von Kaisenberg et al. found increased levels and an aberrant distribution of two chondroitin sulphates in foetal TS skin samples, while the X chromosome encoded proteoglycan biglycan, unsurprisingly, was underexpressed to half of the values [34]. Chondroitin sulphates, as part of proteoglycans, major components in the architecture of an arterial wall’s intima, are of leading importance in the regulation of the formation, and thus strength, of collagen. Biglycan, which is not only able to bind and store growth factors and activate vasculogenesis [35], but plays an important role in the correct formation of collagen fibrils [36], is also richly present in the aortic wall. Therefore, both structural alterations are possible etiological explanations for aortic abnormalities in patients with TS leading to higher rigidity.

Our study must be appraised in the light of its limitations. We did not exclude patients with CoA and CoA repair, since our study population exhibited a similar percentage of CoA as reported in the literature, and its implications are an important part of the syndrome. While the effect of HRT and GHT on blood pressure, and thus, elasticity is still subject to discussion [37], its impact could not be investigated, as it is standard of care in pediatric TS. Echocardiography is known to be highly dependent on the expertise of the examiner. We sought to address this by having one experienced pediatric cardiologist conduct all evaluations, keeping in mind that echocardiographic view quality is far superior in children and adolescents than in adults and that the alternative choice, magnet resonance imaging, is notoriously problematic in children because of a high heart and breathing rates and difficulties regarding breathing instructions as well as frequent non-compliance in younger children, often leading to necessity of sedation.

In conclusion, with this prospective study, more insights into the intrinsic aortopathy in TS were gained. The elasticity of the aorta deteriorates as early as in childhood, unrelated to aortic dilatation and congenital heart and valve defects, reaching levels comparable to those in age-matching BAVs and Marfan syndrome [15, 38] patients. While impaired elasticity levels were found in all TS patients, higher rates of dilatation of the AA were more common in the ones with BAVs. Whether the higher stiffness and lesser distensibility in TS are leading to a greater risk of aortic dissection, explaining the unexpected dissection rate of 10% in patients with normal aortic diameters and without further risk factors is still unknown and needs to be answered in additional investigations, but our data points in this direction.